Microphone arrays may include a number of geographically arranged microphone sensors for receiving sound signals (such as speech signals) and converting the sound signals to electrical signals. The electrical signals may be digitized by analog-to-digital converters (ADCs) for converting into digital signals which may be further processed by a processor (such as a digital signal processor). Compared with a single microphone, the sound signals received at microphone arrays may be further processed for noise reduction/speech enhancement, sound source separation, de-reverberation, spatial sound recording, and source localization and tracking. The processed digital signals may be packaged for transmission over communication channels or converted back to analog signals using a digital-to-analog converter (ADC). Microphone arrays have also been configured for beamforming, or directional sound signal reception. The processor may be programmed as if to receive sound signals from a specific sound source.
Additive microphone arrays may achieve signal enhancement and noise suppression based on synchronize-and-add principles. To achieve better noise suppression, additive microphone arrays may include a large inter-sensor distance. For example, the distance between microphone sensors in additive microphone arrays may range from a couple of centimeters to a couple of decimeters. Because of the large inter-sensor spacing, the bulk size of additive microphone arrays may be large. For this reason, additive microphone arrays may not be suitable for many applications. Additionally, additive microphones may suffer the following drawbacks. First, the beam patterns of additive microphone arrays are frequency-dependent and the widths of the formed beams are inversely proportional to the frequency. Therefore, additive microphone arrays are not effective in dealing with low-frequency noise and interference. Second, the noise component from the additive microphone arrays is generally attenuated in a non-uniform manner over the entire spectrum, resulting in undesirable artifacts in the output. Finally, when the incident angle of the target speech source is different from the array's facing direction (a situation which may often occur in practice), the speech signal may be low-pass filtered, resulting in speech distortion.
In contrast, differential microphone arrays (DMAs) allow for small inter-sensor distance, and may be made very compact. DMAs include an array of microphone sensors that are responsive to the spatial derivatives of the acoustic pressure field. For example, the outputs of a number of geographically arranged omni-directional sensors may be combined together to measure the differentials of the acoustic pressure fields among microphone sensors. Thus, different orders of DMAs may be constructed from omni-directional microphone sensors so that the DMAs may have certain directivity. FIG. 1 illustrates a third-order DMAs. As shown in FIG. 1, the first-order signal differentials of the DMAs may be constructed by subtracting two nearby omni-directional microphone sensors' outputs. Second-order differential DMAs may be constructed by subtracting two nearby first-order differential outputs. Similarly, third-order differential DMAs may be constructed by subtracting two nearby second-order differential outputs. Similarly, an Nth order differential DMAs may be constructed from subtracting two differentials of order N−1.
Compared to additive microphone arrays, DMAs have the following advantages. First, DMAs may form frequency-independent beam patterns so that they are effective for processing both high- and low-frequency signals. Second, DMAs have the potential to attain maximum directional gain with a given number of microphones sensors. Third, the gains of DMAs decrease with the distance between the sound source and the arrays, and therefore inherently suppress environmental noise and interference from far-away sources.
An Nth order DMA may be constructed from at least N+1 microphone sensors. As shown in FIG. 1, the DMA may be constructed in the time domain by directly differentiating the output signals of two nearby microphone sensors at the first-order level or their corresponding derivatives at higher order levels. The implementation as shown in FIG. 1 has drawbacks. For example, each level of differential outputs of the DMA requires equalization filters for compensating the array's non-uniform frequency response, particularly for high-order DMAs. Equalization filters have been difficult to design and tune in practice.
Another drawback is that DMAs may amplify sensor noise. Each microphone sensor may include membranes what may vibrate in response to sound waves to convert pressures applied by the sound waves into electrical signals. The generated electrical signals include sensor noise in addition to the measurements of the sound. Unlike environmental noise, the sensor noise is inherent to the microphone sensors and therefore is present even in a soundproof environment such as a sound booth. Typically, microphone array outputs may have 20-30 dB of white noise due to the sensors depending on the quality of microphone sensors. DMAs are known for amplification of sensor noise; and, the higher order DMAs, the larger the amplification. For example, a third-order DMA of current art may amplify the sensor noise to about 80 dB, rendering the DMA useless for practical purposes.
One way to reduce the sensor noise is to use larger membranes in the microphone sensors. However, both larger membranes and larger microphone sensors increase the bulk size of DMAs. Another way to reduce the sensor noise is to use materials that generate less noise. However, the lower the generated sensor noise, the more expensive the microphone sensors. For example, a 20 dB microphone sensor can be much more expensive than a 30 dB microphone sensor. Finally, no matter how microphone sensors are fabricated, the sensor noise inherently exists and is subject to amplification by DMAs. Thus, the presently available and/or known DMAs are limited to one or two orders of differentials. Accordingly, a need exists to improve over the present DMAs and provide an improved low noise differential microphone array.